Copper and nickel are two of the most commonly used alloying elements in P/M steels. Copper hardens and strengthens steels. It melts during the sintering process and thus relatively coarse copper powders can be used in the steel without impairing mechanical properties. Finer copper powders are desirable in P/M. However, the cost is generally too high for the benefit obtained. Nickel also adds hardness and strength to the steel while providing it with good ductility properties. Because coarse copper powders can be used the cost of adding copper is low compared to nickel. The addition of nickel is made via the use of finer powders since nickel does not melt during sintering. Finer powders permit a better distribution via solid-state diffusion.
The liquid phase sintering of copper has a negative effect in steel since it causes the P/M part to swell. The dimensional swelling of parts containing copper can be quite high causing them to go out of specifications and also lose density. Parts makers often add nickel to copper-containing steel, because the nickel causes densification, which counteracts the swelling caused by the copper.
Alloying powders are generally added to steel master powders (typically iron plus carbon) in two ways: either as admixed powders or as fully pre-alloyed powders. Admixed powders are prepared by mixing the iron or steel powder with the desired alloying element(s) in elementary form. The fully prealloyed steel powders are manufactured by atomizing a steel melt containing the desired composition of alloying elements to a powder. Hybrid powders combine these two alloying methods whereby prealloyed iron powders are admixed with alloy powders.
Admixed powders have a major disadvantage over prealloyed powders because they are prone to: a) segregation (due to the non-uniform composition of components) during transportation and processing; and b) dusting during handling. The former undesirable phenomenon of segregation occurs because the powders consist of particles that often differ considerably in size, shape and density and are not physically interconnected. Thus admixed powders are susceptible to segregation during their transport and handling. This segregation leads to varying compositions of green compacts manufactured from the admixed powders and thus to varying dimensional changes during the subsequent sintering operation and to varying mechanical properties in the sintered state. Another drawback of admixed powders is their tendency to dust especially if the alloying element is present in the form of very small particles.
In fully prealloyed powders segregation is not an issue because every particle has the same composition. Dusting is less of a concern due to the absence of very fine particles. However, prealloyed powders are much less compressible than admixed powders because of the solid solution hardening effect each alloying element has on the host iron powder.
In spite of the drawbacks, the use of admixed powders has certain advantages over fully prealloyed powders. The mechanical properties of P/M steels are directly related to their density which, in turn, is directly related to the compressibility of the powders making up the steel. In addition, admixed powders are more economical. Copper is always admixed in P/M steels while nickel is preferentially admixed to maintain compressibility of iron powder.
Diffusion alloying of elements to iron powder was the first step taken to alleviate the segregation and dusting concerns in powder mixtures. British Patent 1,162,702 disclosed the idea of partially thermally annealing alloying elements. Today iron powder producers make various iron powder products with alloying elements (e.g. nickel, copper, molybdenum) diffusion alloyed to the surface of the iron. These diffusion-alloyed blends are generally considered high-performance materials and are used when high physical properties need to be attained in the final part. While used extensively in Europe where P/M parts tend to be smaller and require higher performance, the cost of these powders is relatively high and their use is not as widespread in North America, where parts are larger and material cost is a more important factor in finished part cost.
An alternative solution to the debilitating segregation and dusting problems posed by admixed powders has been developed more recently. Organic resin agents are used to bind the various particles together. This development has been refined to the point where resin-bonded iron powders can compete on a performance basis with diffusion-bonded iron powders of similar composition. However, reports of some problems with agglomeration of very fine powder additives to iron powders during resin bonding indicate that very careful processing may be required to maintain product quality in some materials. Although less costly than diffusion-bonded iron powders, resin-bonded iron powders impart extraneous handling and processing steps to admixed iron powders and therefore present a material cost penalty for the P/M parts producer.
The first known patent disclosing resin-bonding (also known as binder-treating) was U.S. Pat. No. 4,483,905. Binders were used to significantly improve the bonding of fine additives (i.e. −44 μm Fe—P) to coarse iron powder and to minimize the segregation of graphite (carbon) in large-scale steel blends. The binding agents preferred in the patent were: polyethyleneglycol, polypropyleneglycol, polyvinylalcohol and glycerol due to their chemical and physical stability (ability to keep particles bound without hardening over time) and their ability to be burnt off easily during the sintering operation.
U.S. Pat. No. 4,834,800 identified other agents suitable for binder-treated iron powders using a similar process. The patent focused on the use of water-insoluble polymeric resins as the preferred agents.
U.S. Pat. No. 5,069,714 selected one specific binding agent, polyvinyl pyrrolidone (PVP), which was not mentioned in any previous binder-treatment patents, and describes a solvent-based process for carrying out the binder-treatment process.
Currently, standard nickel-copper P/M steels are prepared by placing iron powder, graphite carbon, nickel powder, copper powder and lubricant powder in the appropriate ratios by weight (usually 1-4% nickel, 1-3% copper, 0.2-1.0 graphite, 0.75% wax, balance iron) into a container and mixing the resultant powder mixture until well blended (usually 30 to 45 minutes for a total powder mass up to 10 tonnes).
Alternatively, the P/M industry employs the use of bonded iron powder products, such as high performance diffusion-bonded iron powders and resin-bonded iron powders. In these materials iron and the alloying elements have already been combined, so only lubricant and graphite carbon are added to the blend prior to consolidation into a green part. Some commercial hybrid iron powder products have some of the alloying elements prealloyed such as molybdenum, chromium and manganese, while other elements are admixed (graphite), diffusion-bonded (Ni, Cu, Mo), or resin-bonded onto iron (Ni, Cu, graphite carbon).
The powder mixture is then compacted (typical pressures of 400-700 MPa) in a die to form a green compact and then the compact is sintered at elevated temperatures (1100-1250° C.) for 2045 minutes in a reducing atmosphere (e.g. 95/5 N2/H2).
Studies done by some of the present co-inventors (Singh, et al. “Nickel-Copper Interactions in P/M Steels.” Advances in Powder Metallurgy & Particulate Materials-2004, Metal Powder Industries Federation, December 2004, presented at the June 2004 International Conference on Powder Metallurgy and Particulate Materials in Chicago, Ill.) have shown that improving the distribution of nickel in nickel-copper steels via the use of finer nickel powder also improves the distribution of copper. As copper melts during sintering of steels, the affinity of nickel and copper for each other affects the distribution of copper in the sintered steel. Overall, the improved distribution of nickel and copper obtained with finer nickel powder gives better properties in the final steel part, including significantly improved dimensional control (reduction in part swelling and reduction in part-to-part variation of size change), and improved mechanical properties (higher flexural strength, hardness, tensile strength and lower part-to-part variation of mechanical properties).
Finer nickel powders therefore provide a means for increasing the interaction between nickel and copper as well as improving the distribution of these alloying elements in the sintered steel. While standard grades of copper powder used commercially in the ferrous P/M industry are relatively coarse (eg. −165 mesh) compared to nickel, the benefits in using a finer copper powder are well known. Large pores left by coarse copper powder after melting during sintering of steels negatively impacts on mechanical properties, particularly the dynamic properties of steels. However, as noted previously, the cost of atomized copper powder increases dramatically as the mean particle size approaches 10 micrometers due to low yield. Iron powder producers have circumvented the high cost of fine copper powders in diffusion-bonded iron powder products by employing fine copper oxide and coreducing during the diffusion bonding process. Fine copper oxide can be made economically, as brittle materials can be readily ground to fine particle size. However, fine copper oxide powder has not been used in admixed or resin-bonded iron powders due to poor compressibility and the need for additional carbon to reduce copper during sintering, lowering green density of the compact. While relatively coarse oxide reduced copper powder is commonly used by the P/M industry, there does not appear to have been any attempt to reduce fine copper oxide powder prior to incorporation in either admixed or resin-bonded iron powders, presumably due to caking of the reduced powder and loss of discrete particles, as well as the additional cost and complication of an additional processing operation.
The benefit in the use of fine nickel and copper powders in P/M steels has been demonstrated. However, there is an additional benefit that has been observed in the development of the present invention by placing nickel and copper powders in close proximity to each other. When present in relatively low quantities in the steel, typically less than about 4 wt % Ni and 2 wt % Cu, the opportunity for nickel and copper to interact with each other is limited to the migration of liquid copper to solid nickel during the latter stages of the sintering process. In admixed powder steels, the simple order of addition of powders to the blender can have an effect on the interaction between alloying elements. As part of the present invention, by premixing nickel and copper powders the inventors obtained improvements in properties of sintered steels compared to standard admixing, whereby constituent powders are added at the same time and then blended.
The present invention seeks to provide a means by which this interaction between nickel and copper particles can be enhanced. In particular, by increasing the proximity of nickel and copper particles through the provision of a stable, transportable nickel-copper powder this desired interaction can be further increased.
There is therefore a need for a bonded nickel-copper powder additive for P/M steels that enhances the properties of the P/M steels while eliminating the difficulties posed by current admixed powders or pre-alloyed iron powders.